Field of the Invention
The present invention relates to a negative electrode terminal for a lithium ion battery, having a plurality of layers of different metal materials bonded to each other, a cover member for a lithium ion battery including the negative electrode terminal, and a lithium ion battery employing the cover member.
Description of the Background Art
In batteries, there are a primary cell, a secondary cell, and a fuel cell, all of which are chemical cells, a solar cell, which is a physical cell, and so on. The primary cell cannot be charged and includes a dry cell such as a manganese cell, a lithium cell, a button cell, etc. The secondary cell can be charged and includes a nickel-cadmium battery, a nickel-hydrogen battery, a lithium ion battery, etc. These various batteries are employed according to purposes. For example, the lithium ion battery has such characteristics that there is no high energy density (energy extracted per unit mass or per unit volume) and little memory effect (a degradation phenomenon in which the capacity appears to be reduced when a secondary cell is recharged before the secondary cell is sufficiently discharged) and is widely employed in a portable device such as a portable telephone or a notebook computer.
In recent years, a large-sized lithium ion battery has been employed in an electric vehicle, a hybrid electric vehicle, a smart grid, etc. In these fields, a plurality of lithium ion batteries may often be electrically connected to each other to be employed in order to obtain a large current. A connection component (bus bar) between terminals made of a Cu-based material having low electrical resistance and generating less Joule heat is employed to connect between terminals of these batteries. Conventionally, the bas bur is often mechanically fastened to the battery terminals by a bolt and a nut or the like in order to be capable of easily replacing the batteries in consideration of variations in the characteristics of the individual batteries.
A recent lithium ion battery is disclosed in Japanese Patent Laying-Open No. 2011-210725 (Patent Document 1), for example. In this battery, Al capable of suppressing a chemical reaction with a positive-electrode active material is employed in a collector of a positive electrode, and Cu having low electrical resistance is employed in a collector of a negative electrode. As materials for terminals provided to be exposed from the battery, materials easily welded to the collectors are selected, and Al is employed for a position electrode portion while Cu is employed for a negative electrode portion. Most materials for individual parts of the lithium ion battery are selected in this manner. Furthermore, the structure of employing Al in a portion of a positive electrode portion (a lower portion of a terminal) connecting with a collector made of Al and containing Cu in a portion of the positive electrode portion (an upper portion of the terminal) connecting with a bus bar in order to reduce resistance (contact resistance) resulting from connection between the positive electrode portion made of Al and the bus bar made of a Cu-based material is disclosed in Japanese Patent Laying-Open No. 2010-097769 (Patent Document 2), for example.
In recent years, the quality of a lithium ion battery has been improved by suppressing variations in electrical characteristics, and it has been considered that welding is applied to connect a bus bar to a battery terminal in order to further reduce the weight, reduce the size (reduce the volume), and improve productivity. In addition, it has been considered that a bus bar made of an Al-based material having a smaller density (specific gravity) and enabling reduction in weight is applied instead of a conventional bus bar made of a Cu-based material. According to the structure disclosed in the aforementioned Patent Document 2, for example, the bus bar made of the Al-based material can be easily welded to the positive electrode portion made of Al.
However, when the bus bar made of the Al-based material and the negative electrode portion made of Cu are welded to each other, a reaction resulting from heat in welding occurs, and an intermetallic compound of Al and Cu having weak mechanical strength is generated on a bonding interface by a gradient of composition. Thus, the bonding strength is reduced. In order to solve this problem, the structure of welding a bus bar made of an Al-based material to a terminal member (negative electrode terminal) made of Al and welding the terminal member to the negative electrode portion made of Cu through a two-layered clad material made of Al and Cu is proposed in the aforementioned Patent Document 1, for example.
According to the aforementioned Patent Document 1, the two-layered clad material made of Al and Cu is applied. However, although sufficient bonding strength is previously provided to the clad material, the clad material is an assembly of an Al-based material and a Cu-based material. Therefore, when the heat input in welding is increased and heat of about 500° C. is transferred, there is such a problem that the aforementioned reaction occurs on a bonding interface between an Al layer and a Cu layer of the clad material to generate an intermetallic compound and the bonding strength is reduced. In addition, two components of the terminal member (negative electrode terminal) and the clad material are required to connect the bus bar to the negative electrode portion, and hence there are such a problem that production efficiency is reduced, such a problem that manufacturing costs are increased, and such a problem that these components influence the size and weight of the battery.